辐射定律实验报告

实验八 辐射定律

实验目的：

验证辐射定律 预备知识

热量的传递方式 黑体的概念

热电偶的测量原理

实验概述

我们将物体的各种温度所释放出来的辐射称为热辐射，所有的物体都对外界释放这一辐射能，也从外界吸收此种辐射。如果一个物体比它的外界热，它就会因释放出來的能量比吸收的能量多而变冷，一般情况下受热的固体辐射出的电磁波，其强度及强度对频率的依赖关系与温度及固体的性质都有关。但是当物体处于热平衡状态，热的吸收率和放射率一样的時候，它对电磁波的吸收和辐射达到了平衡，电磁辐射的特性将只取决物体的温度，与物体的其他性质无关。即斯特藩－玻耳兹曼定律：

4T J u σ=

（u J ：辐射通量密度，单位如果一个物体在任何温度下都能够把投射到它上面的任何频率的电磁波全部吸收，这个物体就称为绝对黑体。带小孔的空腔可看作一个绝对黑体，故空腔中的电磁辐射也称为黑体辐射。

图1

实验原理

All bodies radiate heat. The intensity of this thermally excited electromagnetic radiation increases with the temperature of the body, and is also dependent on the surface of this body. At a given wavelength, the more heat a body radiates, the better it can absorb this radiation.

A body which completely absorbs heat radiation of all wavelengths is called a black body . It was Kirchhoff who first proposed using a cavity as a virtually ideal black body. The black body has the greatest absorption factor, and thus, at a given temperature and wavelength, the highest possible emissivity as well.

The Stefan-Boltzmann law states that the total emitted radiation of a black body increases proportionally to the absolute temperature T raised to the fourth power. More precisely, the radiant exitance M, i.e. the total power radiated on one side of the surface with reference to the area of the radiating surface, is defined as M =σT 4 (I) (σ= 5.67•

10–8

2

m w K -4Stefan-Boltzmann constant)

At the same time, the black body absorbs radiation from its environment. Thus, we do not measure the total radiated radiant exitance M, but rather the radiant exitance M’ withdrawn from the black body by radiation. The radiant exitance absorbed from the environment is

M 0 =σT 04 (II) Therefore, it follows that M’ =

σ

(T 4 – T 04) (III).

In this experiment, an electric oven with a black body accessory is used as the “black body”. The black body accessory consists of a burnished brass cylinder and a screen. The brass cylinder, which is sealed at one end, is slid into the electric oven and heated to the desired temperature. The screen, which can be water-cooled if necessary, is arranged in front of the electric oven, so that essentially only the thermal radiation of the burnished cylinder is measured, and not the outer wall of the hot oven. An NiCr-Ni temperature sensor is used to measure the temperature at the brass cylinder.

The thermal radiation is measured using a Moll’s thermopile to which a microvoltmeter is connected. The thermopile contains a number of thermocouples connected in series. The measuring points absorb the incident radiation almost completely, while the comparison points are at the ambient temperature. We can thus take the output voltage of the thermopile as a relative measure of the radiant exitance M ＇。 附：微伏表的使用

Compensating the instrument: Set function switch 3 to position "V". When the measuring circuit is connected but voltage-free, press key 8 for automatic compensation of external and internal offset voltages. If the zero setting of the digital display and analog output is not achieved when the key is released, carry out fine adjustment using the potentiometer 7. 3.2 Voltage measurements Set function switch 3 to position "V".